1. Field of the Invention
The present invention is directed to a method of enhancing the joint between two metal members. More specifically, the invention is directed to strengthening metal members welded at multiple locations along the joint by filling the gaps between the metal members and reacting the metal members metallurgically with a solder.
2. Description of the Related Art
Most automobile bodies are manufactured by welding a series of stamped metal members. This construction is known as `unibody`. The dynamics of the vehicle are related to the structural integrity of the metal joints. It is desirable to manufacture a vehicle having a high stiffness to provide better ride and handling characteristics. One component of this stiffness is the flexing between the metal members at seams and joints. Various methods have been pursued to increase the stiffness of these joints and seams. The metal may be made thicker to reduce the flexing or bending. However thicker metal increases the weight of the vehicle and the cost of the material. It is also possible to increase the number of welds on a particular component to reduce its flexing. However, the increased number of welds significantly complicates and slows the manufacturing processes for producing the vehicle.
It has been proposed to place an adhesive between these metal joints to provide a bond along the entire length of the joint. The adhesive is placed between mating surfaces of the members. The members are brought together and squeeze the adhesive therebetween. Use of structural adhesive has several disadvantages. The adhesive must be applied before the welding operation thus requiring an additional step. The adhesive creates a nonmetallic layer between the two metal members. Welding requires intimate contact between the metal members and thus metal particulates must be added to the adhesives to assure metal to metal contact. This makes it more difficult to weld the members. The welding operation itself creates temperatures in excess of 2,000.degree. F. This vaporizes any nearby adhesive. The adhesive requires post-curing at an elevated temperature for 0.05 to 5 hours which slows the assembly process. The adhesive is also messy and complicated to apply between the metal components. These adhesive joints are difficult to repair. Finally, the adhesive is likely to be more expensive and adding weight to the vehicle. It is desirable to find an alternative to structural adhesive that provides the same degree of stiffening but at a reduced cost and without unduly complicating the welding process.
It has long been known that joint seams or dents in metal members may be filled with a low melting metal composition. Historically, lead-based solder alloys have been used as a body filler for joints and dents. However, due to the environmental hazards of lead particles and the resulting possible legislation of reducing or banning Pb usage, lead-based solder fillers are no longer used in the body assembly.
More recently, a lead-free body filler has been disclosed as described in U.S. Pat. Nos. 4,248,905, 4,358,884, and 4,374,904. These patents teach a lead-free solder composition for application on a steel surface. The solder has a relatively low melting point (about 210.degree. C.) and is spread as a thixotropic paste. The solder is used to fill joint seams and dents in a vehicle body. The solder is made from between 10 and 25 weight percent copper, zero to 7.5 weight percent zinc, zero to 1.5 weight percent iron, and the balance tin. Because this solder is intended for use as a dent or seam filler, the patents caution against preparing a composition that has an excessive liquid base. The excessive liquid base makes the solder too fluid to handle when being applied as a thixotropic paste to the surface of the metal. The solder is made to have a thick and pasty constituency. It may be easily shaped with a paddle on the body and smoothed into the desired surface configuration. However, the compositions that are useful as a body filler, are not useful for filling the small gaps between the metal members. Low viscosity solders are required to wet the metal surface or wick between gaps typically less than 0.001 in.
It is an object of the present invention to enable the use of welded metal members to produce a stiff automotive vehicle body structure. It is also an object of the present invention to enable the use of existing welding equipment and procedures to weld the metal members without the use of adhesives. Finally, it is desirable to produce a metal assembly that has a bond along the entire joint without increasing the number of welds or using adhesives.